Photochemical, thermal, and base-induced access to hydroazulene derivatives via two-carbon ring-enlargement reactions of condensed electrophilic cyclobutenes

Enamines, silyl enol ethers, and P-keto ester anions derived from bicyclo[3.3.0]octan-2-one efficiently underwent a formal [2 + 2] cycloaddition reaction with DMAD and ethyl propynoate leading to a large variety of electrophilic cyclobutenes. The latter were transformed into polyfunctionalized bicyclo[5.3.0]decane (or hydroazulene) ring systems in high yields by fragmentation of the cyclobutene moiety. These two-carbon ring-enlargement reactions were utilized as a synthetic tool for the construction of a polyfunctionalized hydroazulene derivative that represents a potential precursor of the tricyclic framework of ingenol.